powerpc/spufs: Fix build warning when CONFIG_PROC_FS=n

arch/powerpc/platforms/cell/spufs/sched.c:1055:12: warning: ‘show_spu_loadavg’ defined but not used [-Wunused-function]
 static int show_spu_loadavg(struct seq_file *s, void *private)
            ^~~~~~~~~~~~~~~~

Move it into #ifdef block to fix this, also remove unneeded semicolon.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220308100928.23540-1-yuehaibing@huawei.com